The present invention relates to metal vapor and gas discharge devices, and more particularly, to an electrode and its mounting arrangement for use in discharge devices.
Gas and metal vapor discharge devices are well known and each comprises oppositely positioned electrodes that cause the metal vapor or gas, confined within a near vacuum, to be electrically excited to create optical emissions, such as light or lasing. The selected metal, liquid, solid or gas, serves as the source for generating electromagnetic radiation, coherent in the case of lasing, in the ultraviolet, visible, or infrared regions of the spectrum. The metal, liquid or gas is sometimes referred to herein as being the light source. Similarly, the discharge device housing the electrodes and containing the light source is sometimes referred to herein as being a containment cell. Furthermore, the oppositely positioned electrodes are sometimes referred to as being cathode and anode electrodes, but are also referred to herein as simply being cathode electrodes because of the pulsed operation of the discharge device.
The excitation of the metal vapor or gas contained in a near vacuum is commonly created by applying an electrical pulse of several kilovolts at several hundred amperes current between the electrodes. For such an application, it is desired that the arc condition created by the pulsed excitation be confined between the tips of the electrodes and that each of the electrodes be non-sputtering. However, these conditions are not always attainable, as we realized in our initial attempts to provide an electrode for a discharge device.
In our first attempt, the electrodes were made of titanium in a simple cylindrical geometry, attached to an Oxygen-Free Conductivity (.gtoreq.99.9%) (OHFC) copper gasket to form a vacuum tight seal between two end flanges of a partially evacuated, electrically insulated containment cylinder or cell which confined the light source. These electrodes were subjected to operating temperatures in excess of 1000.degree. C. causing thermal expansion thereof so that the electrodes swelled, which in turn caused cracking of the electrically insulating (ceramic) containment cell. In addition, the titanium was found to undesirably sputter. Furthermore, scorch marks occurred along the length of the electrodes, which indicated that the arc or discharge condition was being initiated over the full length of the electrode, rather than being confined between the tips of the electrodes.
Our next electrode design consisted of two concentric, hollow cylinders, brazed together at one end to form a narrow, elongated cup, and attached to an OHFC copper gasket to form a vacuum tight seal between two flanges of the cell. The inner cylinder was composed of thoriated tungsten and the outer cylinder was composed of titanium. The containment cell experienced a crack during high-temperature operation and the electrodes were inspected. Such inspection revealed that scorching occurred along the length of the cylinders, again indicating that the discharge condition was initiated over the full length of the electrode. Further, the outer titanium cylinder was found to sputter at operating temperatures, and the electrode swelled, causing stress fractures in the containment cell.
A further electrode design consisted of a hollow-cathode, thoriated-tungsten electrode brazed to an OHFC copper gasket so as to form a vacuum-tight seal between two flanges of the containment cell. After initial testing at operating temperatures in excess of 1500.degree. C., it was found that this electrode design successfully contained the discharge between the tips of the electrode, and that the thoriated tungsten material did not sputter. However, because of thermal expansion at full operating temperature, the outer edge of the electrode tip swelled against the ceramic containment cell, thereby cracking the cell and causing stress fractures in the outer edge of the electrode tips. Because of these stress fractures, eventually the outer edge of the electrode tips cracked off, and when enough of this edge fell off, the hollow cathode design was defeated. That is, the discharge condition was no longer confined to the tips of the electrodes, and the electrodes received scorch marks along their length.
All of our first three approaches had a common disadvantage in that the faces of the copper gaskets had to be filed smooth after each use so as to allow for the creation of a metal vacuum seal. After several filings, the copper gasket became so thin that it had to be replaced by brazing a new gasket onto the base of the electrode; this limited the usefulness of the electrode configuration.
In our next design effort, a split-ring, hollow-cathode, thoriated-tungsten electrode was brazed onto a copper spacer, which in turn was brazed directly onto a vacuum flange that mated to the containment cell. After a relatively extensive amount of testing, the electrodes were removed from the containment cell for inspection. The electrodes successfully confined the discharge condition to occur between the tips of the electrodes, and the material used for the electrodes did not sputter. The splits (or slots) in the ring of the electrode gave the electrode tip enough flexibility that the outer edge of the electrode tip did not crack even after thermal expansion caused the tip to be forced against the containment cell. In addition, these electrodes were made reusable by being brazed directly onto a stainless steel vacuum flange that mated directly to the containment cell. However, major stress fractures developed around the base of the electrode just above the copper spacer during the brazing process required to attach the electrode to both the copper spacer and the vacuum flange. After many uses at operating temperatures, these stress fractures grew worse until the upper section of the thoriated tungsten cracked off near its base. This cracking indicated that a new thermal stress from different thermal expansion coefficients of the copper spacer, the thoriated tungsten and a stainless steel vacuum flange was being manifested in the fabrication process.
It is desired that electrodes be provided that are capable of withstanding the application of pulsed discharge excitation of several kilovolts at several hundred amperes peak currents and confining the discharge condition of the device created by these high currents to occur between the tips of the electrodes.